Previously it was thought the body's immune system was only capable of pursuing viruses outside cells.

But scientists from the Medical Research Council in Cambridge discovered that antibodies are also capable of fighting viruses within cells.

New drugs could exploit this natural immune mechanism to beat previously untreatable illnesses, the researchers suggested.

Informing Patients

Antibodies can fight viruses within cells as well as outside them.

Boosting levels of an intermediate protein may improve immune response.

Drugs to stimulate this effect are still years away, however.

How significant is the data?Earlier work showed an intra-cellular protein called tripartite motif-containing 21 (TRIM21) is capable of binding to antibodies.

A team led by Dr Leo James set out to investigate whether this meant antibodies were capable of acting within cells as well as outside.

To do this, they attached antibodies to the surface of adenoviruses and used them to infect cells. The antibodies were tagged with a fluorescent dye so they could be visualised.

Researchers found the antibodies not only entered the cell still attached to the virus, but attracted the protein TRIM21 to bind to the virus and trigger its disintegration.

The researchers said: 'Because TRIM21 neutralisation occurs before the transcription of viral genes, this offers the possibility of "curing" rather than killing an infected cell.'

They also showed that increasing the level of TRIM21 protein in cells makes the process more effective. Drugs that increase TRIM21 levels could boost immune response to viral diseases like the common cold, winter vomiting and gastroenteritis, they said.

Are new treatments likely?Lead author Dr James said: 'Doctors have plenty of antibiotics to fight bacterial infections but few antiviral drugs.

'Although these are early days, and we don't yet know whether all viruses are cleared by this mechanism, we are excited that our discoveries may open multiple avenues for developing new antiviral drugs.'

'Up to now, we thought the immobilisation and destroying of the virus attacked by antibodies was a straightforward attack on the surface of the virus. We now know the real action actually takes place inside the attacked virus.'

He continued: 'By understanding the mechanism and the process of such an attack we can look for ways of boosting the process.'

Although in principle drugs could strengthen this natural response, the researchers said they are many years from reality.

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